Si Prat (Thai: ศรีปราชญ์) is a legendary Thai poet believed to have served King Narai during the 17th century. According to traditional tellings, he was subsequently banished to South Thailand as a result of his personal indiscretions and executed after having an affair with the wife of a provincial governor.

Si Prat is regarded as one of the foremost Thai poets of the Ayutthaya period and epitomizes the genius court poet during the kingdom's golden age of literature. The 131-stanza poem Kamsuan Samut (c. 1680), regarded as a seminal work from the era, has traditionally been attributed to him, as is the Anirut Kham Chan. However, the attributions have been questioned by late 20th-century literary scholarship, and the historical existence of Si Prat is nowadays regarded as a myth by most academics.

According to tradition, Si Prat was born Si in 1650, the son of the court astrologer, poet, and royal teacher Phra Horathibodi. He demonstrated a flair for writing from a young age. The story goes that one day when Si was twelve years old, his father had been tasked by King Narai to finish a poem for him due to writer's block. Unable to complete the task right away, Si's father brought the work home, and next morning found that the missing lines had been filled in by his young son. Impressed by the sharp writing but also worried that his son had overstepped his place, Si's father reluctantly submitted the poem to the King, who was very pleased and accepted Si into his court as a royal page. One legend has it that during a hunting expedition, Si was accorded the title "Si Prat", which means "great scholar", by King Narai after composing a few lines that pleased him greatly.

Si Prat's career as a court poet coincided with what is dubbed the "Golden Age of Thai literature". In his early years, he is believed to have written the Anirut Kham Chan ("The Tale of Anirut"), a parallel to the story of Aniruddha in the Puranas which has been described by Thomas J. Hudak as a "masterpiece of punning and word play". Many anecdotes tell of Si Prat engaging in improvised oral poetry with members of the court, such as guardsmen and royal concubines, sometimes flirting with the latter. At age 27, he was discovered to have had an affair with Thao Si Chulalak, one of Narai's consorts. This was punishable by death, but out of respect for Si Prat's father, the King commuted his sentence and banished him to Nakhon Si Thammarat in the south instead. On his way to exile, Si Prat is believed to have penned the Kamsuan Samut ("Ocean Lament") or Kamsuan Si Prat ("The Mourning of Si Prat"), a nirat poem which follows the speaker as he flees from Ayutthaya with a heavy heart. It has been described as a masterpiece of Thai literature.

In Nakhon Si Thammarat, Si Prat became established as a poet, and was often invited to the governor's residence. In 1683, at 31 years of age, he was found to be having an affair with one of the governor's minor wives. Ignorant of Si Prat's reputation in the royal palace, the governor promptly had Si Prat executed.

Legend has it that while blindfolded and tied to the post for his beheading, Si Prat used his toe to write a khlong poem in the sand. This poem cursed those who wrongly condemned him to face the same sword. A few months later, Narai, wishing to pardon Si Prat, learned of his death and accordingly had the governor executed with the same sword.

— A common variation of the poem said to be written by Si Prat upon his execution

Bear me witness, ye Earth,
Spite not God's image in man.
If wrong I did, let this sword fall true and sharper,
If wronged I am, let it strike back the striker.